1. Field of the Invention
The present invention relates to the hydraulic control system of an automatic transmission in which a clutch-to-clutch downshift based on the release of a higher-speed-stage side clutch and the engagement of a lower-speed-stage side clutch is effected in a power-ON state.
2. Description of the Prior Art
In effecting the specified gearshift of an automatic transmission installed in a motor vehicle, it is often the case that two clutches (each being a clutch in a broad sense, and including an ordinary clutch and a brake) must be respectively engaged and released at the same time (as a so-called "clutch-to-clutch gearshift"). In this case, unless the engagement and release of the respective clutches are accurately synchronized, the torque of the output shaft of the automatic transmission will fall suddenly, and/or the r.p.m. of the engine of the motor vehicle will rise abnormally.
The official gazette of Japanese Patent Application Publication No. 8665/1994 discloses an example of a control in the case where a clutch-to-clutch downshift is to be effected in a power-ON state. In this example, when the command of the downshift has been issued, the hydraulic pressure of a higher-speed-stage side clutch is first lowered. Subsequently, when the r.p.m. of a turbine have risen owing to decrease in the transmission torque capacity of the higher-speed-stage side clutch, the lowering of the hydraulic pressure of this clutch is stopped. Further, when the deviation between the r.p.m. of the turbine and the synchronous r.p.m. of a lower-speed-stage side has become a predetermined value or less, the hydraulic pressure of the lower-speed-stage side clutch is raised to keep the turbine r.p.m. Thereafter, the hydraulic pressure of the lower-speed-stage side clutch is gradually raised while gradually lowering that of the higher-speed-stage side clutch. Thus, the engagement and release of the clutches are changed-over.
With this control, however, a gearshift shock might be incurred in the latter half of the gearshift operation when, in the course of lowering the hydraulic pressure of the higher-speed-stage side clutch and raising that of the lower-speed-stage side clutch, the hydraulic pressures are deviated from expected design values due to the manufactural discrepancies, secular changes, etc. of the various dimensions of the clutches.